Lithographic printing entails applying aqueous dampening solutions to a printing plate to cover the background areas of the plate followed by application of oil-based inks to those areas of the plate which are to serve as the image areas. As will be better understood from the following detailed description of the invention, the aqueous solutions are generally transmitted from a source to the printing plate by a train or a series of rolls, including a ductor roll and a water form roll. The object is to apply a uniform, continuous, clean ample film of aqueous solution to the background areas of the printing plate over the duration of the printing cycle in as economical and efficient a manner as possible. There are several hurdles to overcome in achieving this multifaceted objective, which involves many inherently antagonistic demands.
The demand for efficiency and economy begins with providing a replaceable surface for a dampener roll, as an integral roll would generally be too costly. The replaceable surface, in the form of a roll cover, should be dimensioned to be readily applied to the roll, and thereafter be capable of fitting snugly on the roll in the presence of rotational and other distortional forces generated during operation. This fitting and gripping problem has been addressed in the prior art, typical of which is the discussion in U.S. Pat. No. 3,180,115. In this patent, roll covers incorporating elastic yarns are described to obviate the shortcomings of the many secondary anchoring means such as adhesives, end-ties and the like which have been previously adopted. A related improvement in elastic dampener roll covers is described in U.S. Pat. No. 4,043,142 wherein a knit fabric is used that reduces the tendency of the fabric to produce a pattern in the printed copy.
A continuous, uniform aqueous dampening film is essential to achieving quality printing. Discontinuities or patterns in the film can be caused by irregularities in the cover surface, resulting from the basic design of the cover (e.g., see the discussion in U.S. Pat. No. 4,043,042 of the elastic cover which is the subject of U.S. Pat. No. 3,180,115) or from distortion or wear of the cover during use. Thus, the cover must be free of inherent irregularities in initial design and have uniform wearing qualities to preclude introducing irregularities in use. Additionally, the cover should have sufficient structural integrity to withstand distortive forces generated in use. Roll covers composed of napped, loosely woven cotton material called molleton present problems in nonuniformity due to the presence of the napped fibers. The fibers tend to compress with time necessitating a break-in period to reach a steady state where uniformity is achievable.
Insofar as providing a clean dampening film there are at least two considerations in the selection of a dampener roll cover. First, the roll cover may be a source of contaminants in the form of lint or loosened fibers. Deposition of lint or loose fibers on the printing plate in the course of dampening application interferes with the subsequent application of ink to the plate and results in a loss in print quality. The second consideration involves contamination of the roll cover with ink, and the ability to remove the ink from the cover. After a period of press operation, a slight, but significant, amount of ink works its way back on the fountain train and deposits itself on the ductor roll cover. The ease with which the roll covering may be cleaned of ink is thus an important criterion in measuring the utility of the cover. For example, when a molleton roll becomes dirty with ink residue, the entire roller assembly must be disengaged and the surface scoured in a separate cleaning bath. The removal and substitution of molleton covers must also take place when changing the color of the ink as is frequently required on single color presses. The covering that is the subject of this invention can be cleaned on press and does not require substituting for changes in the color of the ink.
U.S. Pat. Nos. 3,229,351 and 3,293,097 disclose dampening roll covers comprised of a seamless, cohesive, porous, hydroscopic, non-woven cylindrical sleeve having a uniform surface texture, the sleeve being comprised of hydrophilic, randomly disposed fibers which have certain characteristics of solubility, elongation, and dimensional stability when dry. Examples of such fibers include polyvinyl alcohol (PVA) fibers, polypyrrolidone, rayons, cotton, partially saponified cellulose acetate fibers, and the like. As the ratio of fusible binder fiber or resin (e.g., PVA) to hydrophilic fibers (e.g., rayon) is increased, the strength of the sleeve is increased, but the water holding capacity and permeability decrease. Covers made from these fibers function well as dampening (form) roll covers but have insufficient water holding capacity and permeability for use as a ductor roll cover when formulated to have sufficient strength to maintain their structural integrity over long periods of use.
The ability to provide an ample quantity of aqueous dampening solution relates to the liquid retention properties of the roll cover. Even though the actual source of liquid is contained in the fountain tray, the degree of control for wetting the background surface of the printing plate resides in the effective functioning of the ductor roll. For this reason, a covering for the roll must have the ability of absorb moisture from the fountain roll, to retain sufficient moisture to satisfy the demand and to release controlled amounts of fluid so that the water form roll is sufficiently supplied.
U.S. Pat. No. 4,339,858 discloses a cover for a dampening roll having the above-noted characteristics comprised of a cylindrical shaped article of substantially uniform cross-sectional dimension having inner and outer concentrically positioned layers, the inner layer comprising a water wickable textile fabric and the outer layer comprising a porous, smooth, continuous-surfaced, water-permeable, hygroscopic, cohesive, nonwoven member of hydrophilic randomly disposed fibers, bonding means for bonding the inner layer to said outer layer along the interface of the inner and outer layers, the cylindrical article having a first radial dimension when dry, a second radial dimension when wetted in the presence of a radially expansive force, the second radial dimension being greater than the first radial dimension, the article being capable of retaining the second radial dimension upon drying, and a third radial dimension upon rewetting, the third radial dimension being smaller than the second radial dimension. The textile fabric of the inner layer is relatively costly, and its use causes excessive migration of the PVA binder during forming, which results in decreased strength of the outer layer.